Dialkylbenzenes, including xylene, diethylbenzene and ethyltoluene, all have three sorts of isomers, namely p-, o- and m-dialkylbenzene. As their industrial application, generally speaking, they can be used as organic solvent, resin bridging agent and can serve as a starting material of other specialized chemicals. Generally speaking, p-dialkylbenzene has a broad spectrum of usage more than their isomers. p-diethylbenzene can be directly used as the releasing agent in the course of separation of xylenes and therefore, it is a chemical with potent high added value. The dehydrogenated product of p-methylstyrene can be polymerized, and the so-obtained polymer can in many respect displace the polystyrene, thus lessening the consumption of benzene and alleviating the shortage of benzene. Meanwhile, poly-p-methylstyrene uses directly the toluene as raw material, thus saving the consumption of hydrogen in the course of producing benzene from toluene through the process of hydrogenation and dealkylation and thus lowering the cost.
Dialkylbenzene can conventionally be synthesized by using the existing alkylation catalyst e.g, AlCl.sub.3 --HCl and the unmodified ZSM-5. However, the conventional catalyst is void of selectivity to isomers. Therefore, the isomers of the aforesaid o-, m-, p-dialkylbenzenes can be resulted according to the ratio of thermodynamics. For instance, ethyltoluene is generated by the alkylation of ethylene and toluene. The equilibrium ratio in the thermodynamics is given as follows:
o-ethyltoluene: m-ethyltoluene: p-ethyltoluene=16.3%: 49.9%: 33.7% (600.degree. K.)
As the boiling point of isomers closely approximates to one another, the relative volatility comes near 1. The separation is therefore difficult and the cost of operation is quite expensive. If AlCl.sub.3 --HCl is used as the catalyst, not only separation is difficult but it is also impossible to avoid the loss of raw material due to multiple alkylation. Moreover, such a catalyst is liable to cause serious problems of pollution and corrosion due to its strong acidity.
The Mobil Petroleum Corporation of the United States developed, in 1972, a new type of catalyst, which is known as ZSM-5. The particulars of the method of production were disclosed as per U.S. Pat. No. 3,702,886. The crystal structure of ZSM-5 catalyst has a specific order of arrangement and is a porous aluminum silicate crystal. The pores of this sort of catalyst has a homogeneous pore size and, therefore, some smaller than average particles are adsorbable and larger particles are repelled. Hence, it would be frequently referred to as "molecular sieve". There are many precedents in the industry in making use of this characteristics to conduct chemical reactions. The ZSM-5 catalyst is characterised by its selectivity, being able to satisfy the needs for high selectivity to product of different molecules, but its selectivity still falls short of expectations in the way of isomers of the same kind of product. For instance, when methyl alcohol and toluene are reacted over ZSM-5 catalyst, both the rate and the selectivity is very high for xylenes, but the ratio among the isomers of o-, m-, p-xylene remains near the equilibrium value in thermodynamics (refer to W. W. Kaeding, C. Chu, L. B. Yound, B. Weinstein, S. A. Butter, Journal of Catalysis, 67, p.159, 1981).
The ZSM-5 catalyst is a zeolite with high contents of silicon dioxide. Its silicon/aluminum ratio ranges from 20 to 10,000 or more. Its crystal system is (TPA).sub.2 0--Na.sub.2 0--K.sub.2 O--Al.sub.2 O.sub.3 --SiO.sub.2 --H.sub.2 O. In preparing this kind of catalyst the water solution containing tetra-propylammonium compound, sodium oxide, aluminum oxide, silicon oxide, is usually employed in a specific ratio, the composition is hermetically sealed for heating at 100.degree. C. to 175.degree. C. for a period of from six hours to 60 days to make possible the crystallization. The size of crystal particles, the ratio and specific characteristics of ingredients vary with the conditions of the reaction. The cation of the primitive catalyst such as TPA.sup.+, Na.sup.+ and K.sup.+ can be transferred into some other cations through the well-known techniques of ion exchange. The usually used substituants for cation are hydrogen ion, rare earth element ions, manganese ion and the ions of the Group II or VIII elements in the periodic table.
As commonly known in the art the modification of catalyst could be accomplished by means of impregnation, poisoning of the active sites on external surface and steam treatment. Among these treatments, the impregnation process is most effective. The term "impregnation process" refers to the immersion of a ZSM-5 catalyst in a solution containing magnesium, phosphorus, boron or silicon- compounds and then air-dried and calcined to convert the compounds into oxides, thus obtaining a modified catalyst. Take, for example, the modification through the magnesium impregnation. The ammonium-contained NH.sub.4 ZSM-5 catalyst or hydrogen-contained HZSM-5 catalyst obtained after ion-exchanging is slurried in aqueous solution of magnesium acetate and is then heated at 70.degree.-95.degree. C. and allowed to stand overnight. The viscous solution is placed in a crystalizing dish and dried in an oven at a relatively high temperature of about 120.degree. to 200.degree. C. for about 10 hours. Finally it is calcined overnight at a temperature as high as about 500.degree.-550.degree. C. As a result, the modified catalyst through immersion with adequate contents of magnesium is obtained. For phosphorous-modified catalyst, the solution of phosphoric acid, diammonium acid phosphate or trialkyl phosphine is used. the boiling point of such a solution is low. Therefore, the immersion is usually carried out at room temperature for a longer than average period of time, or otherwise, it may be distilled for a longer than average period of time with reflux column. Then, it is over dried slowly and calcined later on. However, the catalyst as modified through impregnation is reduced by far in activity and regenerability as demerits. Besides, during the synthesis of ZSM-5 catalyst, a slight difference in the operational conditions (such as the feeding of material, mixing of temperature, etc.) may oftenly result in significant difference in the performance of ZSM-5 catalyst, for each lot of the yield. Therefore, the modification of the catalyst through impregnation may oftenly make it impossible to render a judgment of the optimum amount of the modifier.